RU1790626C - Powdered charge for thermodiffusive chrome-plating of steel products - Google Patents

Abstract

The inventive charge contains, wt.%: Dusty waste from the smelting of chromium 45-74, ammonium iodide 2-5, aluminum powder 10-20, aluminum oxide - the rest. 2 tab.

Description

The invention relates to chemical-thermal treatment of steel products and can be used in metallurgy, engineering and other industries to increase the wear resistance of products from tool and structural and other steels.

A composition is known for saturating the surface layers of steel products with chromium in a powder mixture: chromium - 50%, A 20z-49% IMN4CM%.

The main disadvantages of this powder mixture is the use of highly scarce, expensive metal chromium powder and, as a consequence, its high cost. In addition, after chemical-thermal treatment in a mixture of this composition, metal chromium particles adhere to the surface of the products, which impairs their quality.

The closest in technical essence and the achieved effect to the proposed one is a powder mixture for diffusion chromium plating of steel products 2, which contains, wt.%:

Copper Powder 20-30

Aluminum powder 15-17

Ammonium Chloride 2-3 Chromium Oxide Else Else This metal oxide charge allows to obtain diffusion layers with increased corrosion resistance and hardness on steel products up to 40-60 µM thick.

However, the use of scarce and expensive copper powder, as well as standard chemically pure chromium oxide powder, reduces the charge efficiency. In addition, this mixture results in a coating having a significant porosity and surface roughness, which reduces the quality of the finished product. Thus, the main disadvantages of the prototype are the high cost of the mixture and the low quality of the resulting coating on the finished product. The aim of the invention is to increase the efficiency of the charge and improve the quality of the chrome coating on finished products by reducing porosity and surface roughness.

 This goal is achieved by the fact that in the known powder mixture for thermal diffusion chromium plating of steel

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articles comprising a chromium-containing component, aluminum powder, aluminum oxide and an activator, according to the invention, as a chromium-containing component, the charge contains pulverized oxide waste from ferrochrome smelting, and as an activator, ammonium iodide in the following ratio, wt.%:

Dusty waste from chromium smelting: v: 45-74 Ammonium iodide 2-5 Alumina powder 10-20 Alumina Remaining Use as a chromium-containing component of dusty waste from chromium or ferrochrome smelting, which has accumulated a huge amount in metallurgy, as well as the exclusion of deficient copper powder increases its profitability. Extraction from gas purification cyclones of said powder components does not require additional treatments, except for mixing in a certain proportion and their averaging. Dust from ferrochrome smelting contains the component necessary for diffusion chromium plating - chromium oxide (Cr20z), which, when the mixture is heated, is reduced by aluminum to metallic chromium by the reaction Cr20z + 2A1 A 20z + 2Cg. The remaining oxides (A1203, CaO and SiOa) contained in the dusty waste, due to the high dispersion of the powder mixture upon heating, form calcium and silicon aluminates. The resulting aluminates are inert with respect to the steel billet being processed, thereby fulfilling the role of a filler that improves the gas permeability of the charge.

Ammonium iodide, introduced into the mixture in a small amount, reacts with chromium atoms reduced from chromium oxide and gives the concentration of its volatile compounds, for example, by type

6NhUI + 2Cr 3N2 + 12H2 + 2Crl3 t At temperatures above 900 ° C, the reverse chemical reaction occurs on the surface of the alloyed steel billet: m Crn - (m-n) Cr + Crm where. As a result, chromium is released on the surface, which diffuses the chromium coating of the workpiece.

Thus, ammonium iodide is used as a reaction catalyst and facilitates the rapid delivery of chromium atoms through the gas phase from the deep layers of the charge to the surface of the workpiece. Powder

aluminum acts as a reducing agent of chromium oxides in the entire volume of the charge, and the resulting inert calcium and silicon aluminates are used as effective

gas permeable filler.

All this significantly reduces the porosity and surface roughness, i.e. improves the quality of the chrome coating on products.

The specified mixture is obtained by mechanical mixing of these components.

The change in the content of the charge components has a significant effect on

5 thickness, hardness, corrosion resistance and quality (porosity, roughness of the chrome coating). In a charge with an inert filler made of alumina, the dust content from smelting ferrochrome less than 45% after the saturation of the surface of the product leads to a sharp decrease in the density and thickness of the diffusion layer, reducing its corrosion resistance to 6.2 mg / mm,

5 hardness up to 1820 MPa, and the layer thickness up to 6-9 microns.

This is also facilitated by a decrease in the content of the process catalyst (ammonium iodide) of less than 2% and the reduction (ammonium) of less than 10%. At the indicated concentrations of these components, the mixture contains either an insufficient amount of chromium oxides (less than 30%), or an insufficient amount of volatile

5 compounds of chromium atoms of the Cr3 type, or the reduction ability of the mixture is significantly reduced. These signs cause adverse thermodynamic and kinematic conditions for

0 thermal diffusion saturation, which, in turn, leads to the formation of a chromed surface of products of unsatisfactory quality (high porosity).

5, in a charge with a filler of pure alumina (corundum powder) with a dust content of more than 74% from smelting ferrochrome, after saturation, a decrease in the quality of the diffusion layer is observed. Despite the relatively deep diffusion zone (more than 20 microns), its hardness and corrosion resistance are significantly reduced. A decrease in the quality of the surface is also facilitated by an increase in the aluminum content in the charge of more than 20%. These coating properties are due to the fact that an excessively high reducing ability of the mixture or a high concentration of chromium oxides in it leads to the formation on the surface of the product

chrome zone with low density, high porosity and roughness.

An increase in the content of ammonium iodide by more than 5% increases the cost of the charge and reduces the quality of the coating (its porosity increases).

An increase in the aluminum powder content of more than 20% increases the cost of the charge and reduces the quality of the coating due to alloying with aluminum.

According to the claimed solution, powder blends with various component contents were prepared. The granulometric and chemical composition of pulverized wastes from ferrochrome smelting is shown in Table No. 1. Samples from ShKh-15 steel were subjected to chromium plating. The process of diffusion saturation of the surface of the samples was carried out at 1050 ° C ± 15 ° C for 4 hours in containers with a fusible seal. The container was sealed with boric anhydride with ground glass, which form a fuse during heating of the container. After being kept in an oven, the container was removed, cooled to room temperature and unpacked. For comparison, similar samples were subjected to chromium plating in a powder mixture made according to the prototype. The test data are summarized in table No. 2.

Corrosion testing of the samples was carried out in a 10% aqueous solution of nitric acid at a temperature of 70 ° C. The corrosion rate was estimated by the loss of mass of the sample per unit surface per hour. Weighing before and after the tests was carried out on an analytical balance with an accuracy of 0.001 g.

The hardness of the diffusion layer was determined on a PMT-3 microhardometer, and the thickness and porosity of the coating were determined on a Neohpot2 .. metallographic microscope. ; - ..; .

The surface roughness of chrome-plated products was evaluated using a profilometer.

As shown by tests of the diffusion layer of chromium on the surface of products in some parameters (layer thickness, strength and corrosion resistance) it is not inferior to the prototype, but surpasses it in surface quality (porosity and roughness).

The proposed powder charge in comparison with the prototype has the following advantages: increased efficiency and improved quality by reducing porosity and surface roughness.

Claims (1)

SUMMARY OF THE INVENTION A powder mixture for thermodiffusive chromium plating of steel products, including a chromium component, aluminum powder and an activator, characterized in that, in order to increase its economy and improve the quality of the chromium coating by reducing porosity and surface roughness, the mixture further comprises aluminum oxide, in quality chromium com Ponent - pulverized waste from chromium smelting, and as an activator - ammonium iodide in the following ratio of components, wt.%: Dusty waste from chromium smelting 45-74 Ammonium iodide 2-5 Alumina powder 10-20 Alumina 45 Granulometric and chemical composition of pulverized and powdery waste products Table 1 Table 3